This invention is concerned with the precision mapping of the ocean bottom through the transmission and reception of sonar or sonic pulses called "pings". Currently, such a mapping is usually performed by a single survey ship carrying sonar sounding equipment, traveling along a prescribed course while transmitting and receiving flat fan shaped sonic pulses directed downward, transverse to the ship's longitudinal axis. The location of the ship, the round trip timing and angle of reception of the pulses scattered off the ocean bottom are recorded and further analysis yields level depth contour curves for a portion of the swath of the ocean bottom insonified by these pulses. The swath of ocean bottom mapped in this manner is approximately a region lying symmetrically along and below the survey ship's track. This swath has an angular opening limited to about 30 to 45 degrees on either side of the downward vertical, since wider angled backscattered pulse reflections are often too weak and diffuse to give reliable depth data. Furthermore, if the ocean bottom in some locale is particularly smooth, backscattering of the signal may not provide a distinct enough echo to allow effective measurements, because most of the pulse may be dissipated by being scattered specularly forward away from the vertical.
Shallow water depth measurements with the method now used also prove to be inefficient. The restricted angular opening of the insonified swath in shallow water results in the mapping of such a narrow region that a large number of passes are required to produce continuous map coverage.
On-going internal calibration and adjustment of the depth measurements is also not practical, since only a single beam is transmitted and, therefore, no matching data is available for comparison unless multiple passes are made over the same region.
Further limitations of accuracy result from systemic navigational errors. Until recently, the precision required in the mapping of the ocean bottom was limited by varying uncertainties in positional fixes due to these navigational errors. The inability to sharpen positional fixes precluded the use of more than one ship to map a broader swath while adhering to the required mapping accuracy tolerances, since using more than one ship tended to compound navigational and positioning errors.